Nitride semiconductor light emitting device and manufacturing method thereof

ABSTRACT

A nitride semiconductor light emitting device having preferable light emitting characteristics even if dense threading dislocations extend through single crystal layers. The nitride semiconductor light emitting device includes an active layer obtained by depositing group-3 nitride semiconductors, and a barrier layer disposed adjacent to the active layer and having a greater bandgap than that of the active layer, the active layer having barrier portions which surround the threading dislocations and are defined by interfaces enclosing the threading dislocation and which are made of the same material as that of the barrier layer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a group-3 nitride semiconductor device(referred to as device hereinbelow), and, in particular a nitridesemiconductor light emitting device and a manufacturing method thereof.

2. Background Art

Extensive research is now underway on gallium nitride (referred to asGaN hereinbelow) and related compounds as a material system for ashortwave light emitting device, in particular, a shortwavesemiconductor laser. A GaN-based semiconductor laser device ismanufactured by successively depositing semiconductor single-crystallayers such as (Al_(x)Ga_(1−x))_(1−y) In_(y) N (0≦x≦1, 0≦y≦1) on acrystal substrate.

A metal organic chemical vapor deposition method (abbreviated as MOCVDhereinbelow) is generally used to produce such a single-crystal layer.In this method, source gases containing trimethyl gallium (abbreviatedas TMG hereinbelow) as a group-3 precursor material and ammonia (NH₃) asa group-5 precursor material are introduced into a reactor to react at atemperature within the range of 900-1000° C., thereby depositingcompound crystals on the substrate. A multi-layer structure comprisingvarious compounds can be obtained by changing the ratio of theprecursors fed into the reactor to grow many different layers on thesubstrate.

If the deposited single-crystal layer has many penetrating defects, thelight emitting performance of the device is deteriorated substantially.Such defect is called threading dislocation, which is a linearlyextending defect that penetrates the crystal layer along the growthdirection. Since a threading dislocation acts as a non-radiativerecombination center for carriers, a semiconductor light emitting devicecomprising a layer with many dislocations suffers from poor luminousefficiency. The above mentioned defect is generated due to crystalmisfit strain at an interface between the substrate and an overlyinglayer formed thereon. Attempts to reduce the effect of the misfit at theinterface have been made by choosing substrate materials having similarcrystal structure, lattice constant, and thermal expansion coefficientto those of GaN-based crystal.

A material, which meets the above requirements and has goodcompatibility with a substrate, is a semiconductor crystal itself.However, as for group-3 nitride semiconductors(Al_(x)Ga_(1−x))_(1−y)In_(y)N (0≦x≦1, 0≦y≦1), it is inevitable to usedissimilar materials such as sapphire or the like, because there is nonitride semiconductor bulk crystal which is most suitable for asubstrate. Sapphire has a lattice constant different from that of GaN byabout 14%.

One approach, known as the two-step-growth method, was proposed toaccommodate the misfit at the interface between a sapphire substrate anda Gan-based single-crystal layer grown thereon to reduce the generationof crystal defects in the GaN-based single-crystal layer. This methodcomprises the steps of forming a lower-temperature buffer layerconsisting of aluminum nitride (AlN) on the sapphire substrate at alower temperature within the range of 400-600° C., and then forming aGaN single-crystal layer over the lower-temperature buffer layer.However, the above method has not been completely successful in reducingthe generation of such defects that pass through the GaN single-crystallayer.

Generally, as a dislocation in semiconductor crystals acts as anon-radiative recombination center and is substantially responsible fordegrading the light emitting characteristics of light emitting devicessuch as light emitting diodes and semiconductor lasers, it is desirablethat the crystals in these devices do not includes any dislocations.Therefore, research is now underway toward reduction of threadingdislocations.

A main object of the invention is to provide a nitride semiconductorlight emitting device having good luminescent characteristics.

Another object of the invention is to provide a method for manufacturingthe above nitride semiconductor light emitting device whereby thegeneration of defects passing through a single-crystal layer formed on asubstrate can be reduced.

SUMMARY OF THE INVENTION

The nitride semiconductor light emitting device according to the presentinvention comprises an active layer comprising group-3 nitridesemiconductors, and a barrier layer made of a predetermined material andprovided adjacent to the active layer. The barrier layer has a greaterbandgap than that of the active layer. The light emitting device furthercomprises a barrier portion, or buried barrier portion defined byinterfaces surrounding a threading dislocation in the active layer madeof the same material as the barrier layer.

The nitride semiconductor light emitting device according to the presentinvention has a feature in that the active layer has one of a single andmultiple quantum well structure.

The nitride semiconductor light emitting device according to the presentinvention has a feature in that the predetermined material of thebarrier layer fills up a recess enclosed with the interfaces on theactive layer to smooth surfaces of the recess.

The nitride semiconductor light emitting device according to the presentinvention has a feature in that the barrier portion has one of acone-shape, truncated cone shape, and combination thereof.

The nitride semiconductor light emitting device according to the presentinvention has a feature in that the group-3 nitride semiconductorsingle-crystal layers are of (Al_(x)Ga_(1−x))_(1−y)In_(y)N (0≦x≦1,0≦y≦1).

The nitride semiconductor light emitting device according to the presentinvention further comprises a low temperature barrier layer providedbetween the barrier layer and the active layer, and that the lowtemperature barrier layer is formed of substantially the samepredetermined material as that of the barrier layer at substantially thesame temperature as the growth temperature of the active layer.

The nitride semiconductor light emitting device according to the presentinvention has a feature in that the low temperature barrier layer has alower AlN composition ratio than that of the barrier layer.

According to the present invention, in order to provide a nitridesemiconductor light emitting device comprising an active layer providedby depositing group-3 nitride semiconductor single-crystal layers(Al_(x)Ga_(1−x))_(1−y)In_(y)N (0≦x≦1, 0≦y≦1) and a barrier layerprovided adjacent to the active layer with a greater bandgap than thatof the active layer, a method comprises the steps of forming a pitdefining a recess attributable to a threading dislocation insemiconductor layers formed on a substrate in the active layer ofgroup-3 nitride semiconductors, and depositing a material of the barrierlayer onto the active layer to form a barrier portion surrounding thethreading dislocation and having an interface defined by the sidesurface of the recess.

The method according to the present invention has a feature in that thestep of forming the pit comprises a step of etching the active layerafter the active layer is formed.

The method according to the present invention has a feature in that theetching in the step of etching is terminated when erosion along thethreading dislocation partially reaches the underlying semiconductorlayer.

The method according to the present invention has a feature in that thestep of forming the pit comprises the step of forming the semiconductorlayer at a temperature within a range of 600-850° C. prior to the growthof the active layer.

The method according to the present invention has a feature in that themethod further comprises the step of forming a low temperature barrierlayer of substantially the same material as that of the barrier layer atsubstantially the same temperature as a growth temperature of the activelayer between the step of forming the pit and the step of depositing thematerial.

The method according to the present invention has a feature in that thelow temperature barrier layer has a lower AlN composition ratio thanthat of the barrier layer.

According to the present invention, epitaxial growth of a wafer includesa step of forming pits in the active layer either by etching the waferafter the growth of the active layer is finished, or by growing a partof the guiding layer at lower temperature prior to the growth of theactive layer. The epitaxial growth further includes a step of buryingthe pits which are defined by the interfaces surrounding the threadingdislocations extending into the active layer, with a material having awider bandgap than that of the active layer. Described above, the growthfor a device wafer is completed.

According to the present invention, an barrier portion having a widerbandgap than that of an active layer surrounds a threading dislocation,thereby preventing the diffusion of carriers to the dislocation, so thata device has the improved light emitting characteristics.

BRIEF DESCRIPTION OF THE DRAWINGS

The aforementioned aspects and other features of the invention areexplained in the following description, taken in connection with theaccompanying drawing figures wherein:

FIG. 1 is a schematic, cross sectional view showing a group (III)nitride semiconductor laser device according to the present invention;

FIG. 2 is a schematically fragmentary, perspective view showing anactive layer of the group (III) nitride semiconductor laser deviceaccording to the present invention;

FIG. 3 is a schematically fragmentary, perspective view showing asubstrate or wafer in the manufacturing process of semiconductor laserdevices according to an embodiment of the present invention;

FIG. 4 is a schematically fragmentary, perspective view showing asubstrate in the manufacturing process of semiconductor laser devicesaccording to an embodiment of the present invention;

FIG. 5 is a graph showing current/light output characteristics of asemiconductor laser device according to an embodiment of the presentinvention;

FIG. 6 is a schematic, cross sectional view showing a substrate in themanufacturing process of semiconductor laser devices according to asecond embodiment of the present invention;

FIG. 7 is a schematic, cross sectional view showing a substrate in themanufacturing process of semiconductor laser devices according to thesecond embodiment of the present invention.

FIG. 8 is an electron microscope photograph, of a surface of a wafer inthe manufacturing process of semiconductor laser devices according tothe second embodiment of the present invention;

FIG. 9 is a graph showing an excitation power dependence of emittedlight intensity with respect to a wafer in the manufacturing process ofsemiconductor laser devices according to a further embodiment of thepresent invention;

FIG. 10 is a schematic, cross sectional view showing an active layer ofa group 3 nitride semiconductor laser device according to a thirdembodiment of the present invention; and

FIG. 11 is a schematic, cross sectional view showing a substrate in themanufacturing process of semiconductor laser devices according to thethird embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A group-3 nitride semiconductor laser device according to an embodimentof the present invention will be described below with reference to thedrawings. A first embodiment of the present invention will be describedhereinafter with respect to the case where a wafer in the course ofgrowth is taken out from an epitaxial growth apparatus to be etched asdescribed above.

FIG. 1 shows a group-3 nitride semiconductor laser device according tothe embodiment. The semiconductor laser device comprises a GaN (or AlN)layer 2 formed at a lower temperature, an n-type GaN layer 3, an n-typeAl_(0.1)Ga_(0.9)N layer 4, an n-type GaN layer 5, an active layer 6 ofmultiple quantum well structure including InGaN as a main constituent, ap-type Al_(0.2)Ga_(0.8)N layer 7, a p-type GaN layer 8, a p-typeAl_(0.1)Ga_(0.9)N layer 9, and a p-type GaN layer 10, which aredeposited in the above order on a single crystal sapphire substrate 1.The semiconductor laser device further comprises an n-type electrode 14connected to the n-type GaN layer 3 and a p-type electrode 13 connectedto the p-type GaN layer 10. The p-type Al_(0.1)Ga_(0.9)N layer 9 has aridge stripe portion 18. The device is covered with an insulating film11 of SiO₂ for protection except for the electrodes. In this manner, thenitride semiconductor laser device of the present embodiment comprisesan active layer including a multi-layered structure formed bysuccessively depositing group-3 nitride semiconductors.

The semiconductor laser device emits light through recombination ofelectrons and holes in the active layer 6. The n-type GaN layer 5 andp-type GaN layer 8 act as guide layers. The n-type GaN layer 5 andp-type GaN layer 8 serve to guide light generated in the active layer 6to the guide layers 5, 8. The n-type GaN layer 5 and p-type GaN layer 8also serve to trap electrons and holes into the active layer 6effectively with the larger bandgap than that of the active layer 6. Thep-type Al_(0.2)Ga_(0.8)N layer 7 is a barrier layer for enhancing theconfinement of injected carriers (in particular, electron). The n-typeAl_(0.1)Ga_(0.9)N layer 4 and the p-type Al_(0.1)Ga_(0.9)N layer 9 areclad layers having lower refractive indexes than those of the guidelayers 5, 8, so that differences in refractive indexes between the cladlayers and the guide layers cause light to be guided in a film thicknessdirection. The ridge stripe portion 18 is provided in order to produce astep effective refractive index, by changing the thickness of thecladding layer 9 thereby confining the generated light laterally. Then-type GaN layer 3 is an underlying layer which allows a current toflow. The n-type GaN layer 3 is required because sapphire constitutingthe substrate does not have any conductivity. Additionally, thelower-temperature growth layer, or GaN (or AlN) layer 2 is a so-calledbuffer layer for producing a smooth layer on the sapphire substratewhich is a substance dissimilar to GaN.

Referring to FIG. 2, the above device further comprises a barrierportion 51 defined by an interface 50 of the active layer 6 and formedof the same material as that of the barrier layer 7. The interface 50extends in the vicinity of a threading dislocation 15 extending from thelower guide layer 5 to the upper guide layer 8 through the active layer.

When current flows into the device, electrons 16 are injected from then-type GaN layer 5 into the active layer 6 made of InGaN, as shown inFIG. 2. Since the active layer comprises the well layers 62 which havegreater In composition ratio (that is, narrower bandgap), and thebarrier layers 61 which have smaller In composition ratio (that is,wider bandgap), injected electrons are then collected by the well layers62. Further, injected holes 17 from the p-type GaN guide layer 8 arealso collected by well layers 62 for the same reason. In this case,being blocked by the AlGaN barrier portion 51, both of electrons 16 andholes 17 cannot reach the threading dislocation 15. This is because thethe threading dislocation 15 is surrounded by the AlGaN barrier portion51 having a larger bandgap as compared with that of the well layers 62,which is composed of InGaN with high In composition ratio. Thus, thebarrier portion 51 prevents the injected carriers from reaching thethreading dislocation 15, which usually acts as a non-radiativerecombination, center the device has higher luminescence efficiency thanthat of one having no barrier portion 51.

The device having the structure shown in FIG. 1 will be manufactured inthe following process in which a layer-structure of the laser device isformed on a sapphire A-face substrate by MOCVD.

First, a sapphire substrate 1 is loaded into a MOCVD reactor, and thenplaced in a hydrogen gas flow under a pressure of 300 Torr at atemperature of 1050° C. for ten minutes for thermal cleaning of thesurface of the substrate. The sapphire substrate 1 is then cooled to atemperature of 600° C. Next, an ammonia NH₃ and trimethyl aluminum(abbreviated as TMA hereinbelow) as precursor materials are introducedto the reactor to grow AlN layer, thereby forming a buffer layer 2having a thickness of 50 nm.

Next, after stopping the feed of TMA, the sapphire substrate 1 with thebuffer layer 2 is again heated up to 1050° C. while only NH3 gas isflowing through the reactor. TMG is then introduced to the reactor togrow a n-type GaN underlying layer 3. During the above process,methylsilane (Me—SiH₃) as an n-type dopant is added to the precursormaterial gas.

At a point of time when the n-type GaN underlying layer 3 has been grownto a thickness of about 4 μm, only the feed of TMG is stopped.Meanwhile, methylsilane continues to be supplied with increased flowrate. After this process is maintained for 5 minutes, a feed ofmethylsilane is decreased to a valve required for formation of an n-typelayer. TMG is then introduced again, while TMA is simultaneouslyintroduced so that an n-type AlGaN clad layer 4 is formed.

At a point of time when the n-type AlGaN clad layer 4 has been grown toa thickness of about 0.5 μm, the feed of TMA is stopped, and an n-typeGaN lower guide layer 5 is grown to a thickness of 0.1 μm.

At a point of time when growth of the n-type GaN guide layer 5 iscompleted, the feed of TMG and Me—SiH₃ is stopped, and the substratestarts to be cooled to a temperature of 750° C. At a point of time whenthe substrate temperature reaches 750° C., the carrier gas is switchedover from hydrogen to nitrogen. At a point of time when the gas flowbecomes stable, TMG, trimethylindium (TMI), and Me—SiH₃ are introducedto grow a barrier layer 61.

Subsequently, the feed of Me—SiH₃ is stopped and TMI is increased inflow rate so that the well layers 62 having a greater In compositionratio than that of the barrier layer is grown.

The growth of the barrier layer(s) 61 and the well layer(s) 62 isrepeated in accordance with the number of wells in the multiple quantumwell structure. Described above, an active layer 6 of the multiplequantum well structure is formed.

At a point of time when a barrier layer 61 is formed on the last welllayer 62, the feed of TMG, TMI, and Me—SiH₃ is stopped. At the sametime, the substrate starts to be cooled. At a point of time when thesubstrate temperature is equal to or lower than 400° C., the feed of NH₃is also stopped. At a point of time when the substrate temperaturebecomes a room temperature, the substrate is unloaded from the reactor.

FIG. 3 shows a layer structure on the substrate which is unloaded fromthe reactor in the course of growth without growing of a p-type layer.It has been found that the layer structure includes a lot of threadingdislocations 15, and that the layer structure of the above embodimenthas a dislocation density on the order of 2E9 (1/cm²) by means of ameasurement.

The resultant substrate with the layer structure is then immersed inH₃PO₄ (phosphoric acid) solution heated at 200° C. to be etched. Thus, arecess or pit ascribable to the threading dislocation 15 in the lowerguide layer 5 is formed in the active layer 6.

FIG. 4 shows the substrate after it has been subjected to the etching inthe hot phosphoric acid solution. Since GaN is chemically stable, it issubstantially impervious to etching even in the hot phosphoric acidsolution. However, a proximate portion to the dislocation can beslightly etched to produce a pit 49. When the bottom of the pit thereaches the lower boundary of the active layer, then the pit has themost suitable depth. Thus, the most etching process should be terminatedat the point of time when the erosion along the threading dislocation isreaches the lower guide layer 5. The active layer is supposed to have aplurality of pits 49, each of which has a shape of inverted cone ortruncated cone. When the pits become close, they form a recess ofcontinuous shape.

After the etching process, the substrate is adequately rinsed with purewater, ultrasonically-cleaned with organic solvents, and then is againloaded into the MOCVD reactor.

Then, the substrate is again heated to 1050° C. while NH₃ and hydrogenas a carrier gas are flowing. TMG, TMA, and ethyl-cyclopentadienylmagnesium (Et-Cp₂Mg) as a precursor for p-type dopant are introduced togrow a p-type AlGaN layer 7, thereby forming a barrier layer with athickness of 0.02 μm.

Referring to FIG. 2, at a point of time when the growth of the p-typeAlGaN barrier layer 7 is finished, the pits (recesses) formed by theetching have been filled with the p-type AlGaN. This is because both ofthe high temperature of 1050° C. and the nature of AlGaN (material)promote the surface flattening of the barrier layer 7. Once the smoothsurface of the barrier layer is established, the individual layer to beformed above the barrier layer 7 can be formed with a flat surface.Thus, the barrier portion 51 is formed to have a conical ortruncated-conical shape. It should be noted that though only one barrierportion 51 is shown in FIG. 2, the multi-layer structure may have aplurality of barrier portions, which may be contiguous to one anotherdepending on the configuration of pits.

Subsequently, supply of TMA is stopped, and a p-type GaN layer 8 isgrown on the barrier layer 7 to have a thickness of 0.5 μm. TMA is thenintroduced again so that a p-type AlGaN clad layer 9 is grown thereon tohave a thickness of 0.5 μm. Further, a p-type GaN contact layer 10 isgrown on the layer 9 to have a thickness of 0.1 μm. Thereafter, supplyof TMG and Et-Cp₂Mg is stopped, and cooling is started. At a point oftime when the substrate is cooled to 400° C., supply of NH₃ is alsostopped. At a point of time when the substrate is cooled to a roomtemperature, the substrate with multi-layer structure is unloaded fromthe reactor. The substrate with multi-layer structure of the firstembodiment is referred hereinafter to as a wafer 1.

For the purposes of comparison, a wafer was prepared in the same manneras in the above-described growth method except an absence of the aboveetching process. The wafer intended for the comparison is referredhereinafter to as a wafer 2. More specifically, the wafer 2 wasprocessed in the following manner. After the growth of the last barrierlayer 61, the carrier gas was switched over from nitrogen to hydrogen at750° C. in without unloading the substrate from the reactor. Thesubstrate was simultaneously heated to 1050° C., and the p-type AlGaNlayer 7 and the following respective layers were grown withoutinterruption.

The wafer 1 and the wafer 2 were subjected to a heat treatment in thefurnace to acquire p-type condition at a temperature of 800° C. for 20minutes in nitrogen gas at atmospheric pressure.

Formed on each of the resultant wafers 1 and 2 were a terrace for thep-type electrode, a current path for the n-type electrode, and a narrowridge structure on the terrace for the a refractive index wave guidingand for current constriction.

The narrower ridge structure was formed on the substrate by usingstandard photolithography and reactive ion-etching (RIE) to removeportions except the 5 μm wide ridge leaving the p-type AlGaN claddinglayer 9 with a thickness of about 0.1 μm. Likewise, RIE was then used toremove unnecessary portions including the p-type layers exposing then-type GaN base layer 3 partially.

After removal of the etching mask, an SiO₂ protective layer wasdeposited by means of a sputtering method or the like. A window having awidth of 3 μm was formed in the SiO₂ protective film on the p-type ridgeportion. A window for n-type electrode was formed in the SiO₂ protectivefilm on the exposed portion of the n-type layer.

An n-type electrode 14 was formed on the region on which the n-type GaNlayer 3 was exposed, by depositing Ti (titanium) to a thickness of 50 nmand subsequently Al (aluminum) to 200 nm. The p-type electrode 13 wasformed in the region, where the p-type GaN layer was exposed, byevaporating Ni (nickel) and Au (gold) with thickness of 50 mm and 200nm, respectively.

The wafer processed in this manner was cleaved to form a device shown inFIG. 1. Thereafter, the characteristics of the respective devices weremeasured. The measurement was performed with pulses having a width of0.5 μsec and a duty ratio of 0.02%.

Referring to FIG. 5, the points represented by • indicate current/lightoutput characteristic of a laser device according to the embodiment ofthe invention, which was formed from the wafer 1. The device oscillatedwith a threshold current of about 430 mA at a wavelength of 405 nm. Thepoints represented by ∘in FIG. 5 represent current/light outputcharacteristic of a device formed from the wafer 2, i.e. the comparativeexample. The device oscillated with a threshold current of about 800 mAat a wavelength of 410 nm.

The threshold current of the device according to the embodiment of thepresent invention is to about ½ of that of the comparative sampleshowing the remarkable improvement in the device characteristics.

In an active layer like the present embodiment, injected electrons andholes are mainly collected by the well layers 62 of smaller bandgap inthe case of the present embodiment, those carriers are blocked by theAlGaN barrier portion 51 of the same material as that of the barrierlayer 7 and having a larger bandgap, so that they cannot reach thethreading dislocations 15. As a result, the threading virtually do notact as non-radiative recombination centers. Whereas in the case of thecomparative device injected carriers can freely reach the threadingdislocations, so that the threading dislocations can act as efficientnon-radiative recombination centers to degrade the light emittingcharacteristics.

In an alternative embodiment, the etching process can be performed inthe film forming apparatus. In this case, vapor etching is performed.HCI (hydrogen chloride) can be used as an etching gas. Also,re-evaporation of InGaN layer may be utilized with increasing ofhydrogen flow in the carrier gas while decreasing NH₃ flow rate ascompared with that of the normal growth condition. However, thesemethods are not so effective as that of the above described embodiment.

While the above embodiment adopts an etching as the method for formingthe pits around the threading dislocations in the active layer, a secondembodiment adopts a method of forming the pits in-situ around threadingdislocations in the active layer. That is, the second embodimentutilizes the fact that crystal growth can be inhibited near a threadingdisclocation under particular growth conditions.

Like the first embodiment, the n-type GaN layer 4 and the n-type GaNguide layer 5 were grown on the sapphire substrate 1.

Then, the wafer was cooled to a temperature in the range of 600 to 850°C., for example 770° C. After a switching of the carrier gas fromhydrogen to nitrogen a pit generating layer 5 a of n-type InGaN dopedwith Si was grown to a thickness of 400 Å as shown in FIG. 6 with TMI,TMG, ammonia, and methylsilane as the precursors. During this process,portions 60 where the growth is inhibited were initiated in situ. Inaddition, material of the pit generating layer 5 a is not limited toInGaN but may be a material such an GaN, AlGaN, or the like having abandgap equal to or greater than that of the active layer. Also,non-doped material may be used. Generated at 600° C. or less, howeverwhich is not preferable because of the degradation of layer quality.Further, to establish the growth inhibitition of the portion 60 aroundthe threading disclocation(s), the pit generating layer 5 a is requiredto have a thickness of 100 Å or more, and preferably about 200 Å.However, because of the low growth temperature, the layer qualitybecomes somewhat degraded as compared with the normal crystal layerformed at 1050° C. Also, a pit generating layer 5 a with an excessivethickness might increase waveguide loss.

Subsequently, an active layer 6 was formed on the pit generating layer 5a at 770° C. First, a barrier layer 61 was formed from TMI, TMG, ammoniaand methylsilane as precursors, and a well layer 62 was then formedwhile the feed of methylsilane was stopped and a flow rate of TMI wasincreased. This process was repeated a predetermined number of times togrow a MQW active layer 6 as shown in FIG. 7. In this manner, prior tothe formation of the active layer 6, the pit generating layer 5 a wasformed in a temperature range of 600-850° C., and pits 49 were formedduring the growth of the active layer 6. It should be noted that theactive layer is not limited to MQW.

FIG. 8 shows an electron microscope photograph of a wafer with theactive layer inclined at 45 degrees. The density of pits is estimated tobe 5×10⁹/cm², which is substantially equal to the density of threadingdislocations in the film.

For the purpose of comparision, a wafer was also formed in the samemanner as in the second embodiment except that the low temperature pitgenerating layer 5 a, was not formed.

To compare the light emitting characteristics of the second embodimentwith that of the comparative example, excitation power dependence of theemitted light intensity of the MQW active layer 6 in the wafer wasmeasured. FIG. 9 is a graph showing the results of the measurements withrespect to the excitation power dependence of the emission intensity ofthe wafers in the second embodiment and the comparative example. Anitrogen laser with a wavelength of 337.1 nm was used as the excitationsource. A horizontal axis of the graph indicates the excitation laserpower in percentage to the maximum output. The point denoted by ▪represents the emission intensity of the sample according to the secondembodiment, and the point denoted by □ represents the data of thecomparative sample It was found that the second embodiment (▪) is fiveto ten times superior in light within this intensity to the comparativesample (□) within this measurement range.

The above-described first and second embodiments are characterized bythe pit forming process in which recesses are formed on the threadingdislocations in the semiconductor layer disposed under the active layer.Another important point in applying the present invention to lightemitting devices is to fill the pits 49 shown in FIGS. 4 and 7 withAlGaN to smooth the surface thereof when the barrier layers 7 ofAl_(x)Ga_(1−x)N is formed. In this regard, a third embodiment will bedescribed, which is applied to the above first and second embodimentswith respect to the process of forming the barrier portions.

A temperature of 1000° C. or higher is required in the process forgetting an adequate flat surface. During the rising to this growthtemperature, re-evaporation of the constituent the active layer 6 (thewell layers 62 and the barrier layers 61) tends to occur. Thus, inparticular, the outmost barrier layer 61 in the multiple quantum wellstends to deteriorate.

At the time when the formation of the active layer 6 (that is, the welllayers 62 and the barrier layers 61) of InGaN is completed, a lowtemperature AlGaN barrier layer 71 starts to be grown. The lowtemperature AlGaN barrier layer 71 is a film constituting a part of theAlGaN barrier layer 7. The low temperature AlGaN barrier layer 71 isdisposed utilizing the fact that AlN has much higher thermal stabilityas compared with GaN in the growth ambience. The re-evaporationdescribed above can be effectively prevented by depositing a minutelayer of low temperature AlGaN having an AlN composition ratio of about0.2. The low temperature AlGaN barrier layer 71 preferably has athickness corresponding to several mono-layers, that is, about 20 Å.Excessive thickness of this layer would deteriorate hole injection intothe active layer from the p-type layer. Thus the thickness is preferablyless than 100 Å.

When the structure of the third embodiment is applied to that of thefirst embodiment to form a light emitting device, a wafer is unloadedfrom the reactor, and etched to form pits 49 in the active layer 6,after that or thereafter a low temperature AlGaN barrier layer 71 isgrown.

When the structure of the third embodiment is applied to that of thesecond embodiment to form a light emitting device, a low temperatureAlGaN barrier layer 71 is grown immediately after forming the activelayer 6 without changing the substrate temperature. Thereafter, thecarrier gas is switched from nitrogen to hydrogen, the substrate isheated to 1050° C. for the subsequent film formation.

In either of the above cases, the AlGaN barrier layer 7 is grown in thehydrogen carrier gas after increasing the temperature to 1050° C.

With the third embodiment, the pits 49 are hardly filled up because ofthe low growth temperature of AlGaN barrier layer 71. FIG. 10 depictssuch situation.

In the light emitting device according to the third embodiment, the lowtemperature AlGaN barrier layer 71 has a lower composition ratio of AlNthan that of the AlGaN barrier layer 7. If, if the low temperature AlGaNbarrier layer 71 has a higher composition ratio of AlN than that of theAlGaN barrier layer 7, holes 17 injected from the p-type guide layer(shown by the dotted lines) would tend to be injected into the barrierportions 51 of the AlGaN barrier layer 7 which has a smaller compositionratio of AlN (or a smaller bandgap).

By setting the AlN composition ratio of the low temperature AlGaNbarrier layer 71 to be less than that of the AlGaN barrier layer 7, theholes 17 (shown by solid lines) injected from the p-type guide layer areblocked by the barrier portions 51 as well as electrons 16 injected fromthe n-type GaN layer, thereby not reaching the threading dislocations15.

Accordingly, when the structure of the third embodiment is applied tothat of the second embodiment to form a light emitting device, after thegrowth of the active layer, a low temperature AlGaN barrier layer 71 isformed at substantially the same temperature as that of the growthtemperature of the active layer. A second AlGaN barrier layer 7 is thenformed after the temperature increase. Also, in applying the structureof the third embodiment to any one of the above embodiments, the AlGaNbarrier layer 7 is set to have a larger composition ratio of AlN thanthat of the low temperature AlGaN barrier layer 71.

While the first to third embodiments have been described with respect tolaser devices, the present invention can provide similar effects in thecase of being applied to formation of LEDs (light emitting diodes).

It is understood that the foregoing description and accompanyingdrawings set forth the preferred embodiments of the invention at thepresent time. Various modifications, additions and alternative designswill, of course, become apparent to those skilled in the art in light ofthe foregoing teachings without departing from the spirit and scope ofthe disclosed invention. Thus, it should be appreciated that theinvention is not limited to the disclosed embodiments but may bepracticed within the full scope of the appended claims.

What is claimed is:
 1. A nitride semiconductor light emitting devicecomprising: an active layer comprising group-3 nitride semiconductors, abarrier layer made from a predetermined material and provided adjacentto said active layer, said barrier layer having a greater bandgap thanthat of said active layer, and a barrier portion formed of saidpredetermined material for surrounding a threading dislocation in saidactive layer, said barrier portion being defined by interfaces enclosingsaid threading dislocation.
 2. A nitride semiconductor light emittingdevice according to claim 1, wherein said active layer has one of asingle and multiple quantum well structure.
 3. A nitride semiconductorlight emitting device according to claim 1, wherein said predeterminedmaterial of said barrier layer fills up a recess enclosed with saidinterfaces on said active layer to smooth surfaces of said recess.
 4. Anitride semiconductor light emitting device according to claim 1,wherein said barrier portion has one of a cone-shape, truncated coneshape and a combination thereof.
 5. A nitride semiconductor lightemitting device according to claim 1, wherein said group-3 nitridesemiconductor single-crystal layers are (Al_(x)Ga_(1−x))_(1−y)In_(y)N(0≦x≦1, 0≦y≦1).
 6. A nitride semiconductor light emitting deviceaccording to claim 5, further comprising a low temperature barrier layerprovided between said barrier layer and said active layer, said lowtemperature barrier layer being formed of substantially the samepredetermined material as that of said barrier layer at substantiallythe same temperature as the growth temperature of said active layer. 7.A nitride semiconductor light emitting device according to claim 6,wherein said low temperature barrier layer has a composition ratio ofAlN which is less than a composition ratio of said barrier layer.